The present invention relates to an optical information recording medium on or from which a user can record or reproduce information and, more particularly, to a read/write optical disk having a photosensitive recording layer which is deformed in a beam irradiation area in response to an input light beam modulated to represent recording information and a method of manufacturing the same.
Optical disks on or from which information can be recorded or reproduced by the user are becoming increasingly important as optical information recording media capable of storing a large volume of information and allowing extraction of desired information by random access. In general, in optical disks of this type a hole called a "pit" is formed in a photosensitive recording layer to store data in response to an input laser beam (called a "write beam") modulated to represent information to be recorded. In the data read mode, the pit portion of the medium is continuously irradiated with a laser beam of a predetermined intensity (weaker than a write beam and called a "read beam") so as to reproduce desired information.
Since semiconductor lasers having relatively low power ratings in the vicinity of 20 mW have been used in recent optical disks, low-melting-point metals such as Te, In, Bi, Pb, As or Se are preferable as recording layer metals. However, recording layers of low-melting-point metals have poor oxidation resistance and naturally corrode over a long period of time. When optical disks are left exposed to the air over a long period of time, the recording layer reacts with water and oxygen contained in the air to form an oxide, which significantly degrades the basic functioning of the recording layer. Thus, optical disks having recording layers of low-melting-point metals also have short lives.
When a recording layer comprises a tellurium-carbon (Te-C) film having an extremely high oxidation resistance, the oxidation resistance of the recording layer can be improved and the life of the disk prolonged. A Te-C film has an extremely high oxidation resistance since Te particles in the recording layer are uniformly dispersed in a hydrocarbon matrix. More specifically, the hydrocarbon matrix serves as an oxidation inhibitor for Te particles and improves the oxidation resistance of the recording layer. Empirically, the basic functioning of this recording layer is not substantially degraded even if the layer is left exposed to severe conditions for over 1,000 hours (e.g., at a temperature of 70.degree. C. and a relative humidity of 85%). This indicates that an optical disk having such a recording layer has a life of about 150,000 hours (17 years).
However, an improved optical disk of this type suffers from the problem of low heat resistance. This is attributable to the poor heat resistance of the hydrocarbon matrix. The recording layer of an optical disk is subject to irradiation not only with a write beam for information recording but also with read beams of several different access schemes. In one of the typical schemes, the recording track is traced continuously in a spiral. According to another scheme, the same data is read by jumping to a traced track. In either scheme, continuous irradiation by the read beam results in a rise in temperature in the recording layer. The temperature increase is greater in the latter scheme than the former scheme. When the increase exceeds a given characteristic temperature determined by the type of recording layer used, the layer is subject to deterioration.
In order to prevent such thermal deterioration, the following countermeasures can be taken. When the former access scheme is adopted, the intensity of the read beam is reduced so that the temperature of the recording layer is kept below the characteristic temperature. When the latter access scheme is adopted and a single track is continuously read more than a predetermined number of times, the read beam is automatically returned to its initial position at the center of the disk.
However, since the CNR (carrier-to-noise ratio) of the reproduced signal is proportional to 1/2 the power of the intensity of the read beam, a decrease in read beam intensity leads to a degradation in the CNR. The method of resetting the read beam to the initial position at the center of the disk, on the other hand, causes an increase in data access time. In other words, either method can only be adopted at the cost of the performance of the optical disk.